[Stoves] attempt at swirling secondary air

Tue Apr 29 20:31:35 CDT 2014

Crispin,

Please provide some photos (recent or historic) or links to documents 
about the elongated holes.

Paul

Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD
Email:  psanders at ilstu.edu
Skype: paultlud      Phone: +1-309-452-7072
Website:  www.drtlud.com

On 4/29/2014 6:26 PM, Crispin Pembert-Pigott wrote:
>
> Dear Dave
>
> That is a nice effort and well-reported. Thanks for the small size 
> pictures.
>
> In general, I feel you will get better results and at less cost by 
> using incoming secondary air to achieve the effect.
>
> To get better penetration into the rising column of gas, make holes as 
> follows:
>
> Drill the size you want 1.5mm undersize, maybe 2mm.
>
> Make/find a metal rod the diameter of hole you want.
>
> Sharpen it enough to go into the drilled hole.
>
> Place the sheet metal over a hole drilled into a thick piece of steel 
> - say, 10mm flat bar which can reach inside the stove. The hole should 
> be 3 or 4 mm bigger than the sharpened rod.
>
> Punch the bar through the drilled hole so the rod enters the centre of 
> the hole in the thick bar.
>
> Withdraw it.
>
> Now you have a hole with a radiused edge and a small 'blurted' channel 
> that will direct the air.
>
> If you really really really want to optimise this, the outlet hole 
> should be 1/3 of the area of the inlet and the taper constant, and the 
> length 6 times the inlet diameter. This is hard to make. It has been 
> known since the 1880's.  It applies to all fluids (air is a fluid). I 
> used it in Malawian tobacco barns and it tripled the effectiveness of 
> the 'educting effect' of the combustion gas energy in the chimney. (It 
> means using the chimney to pull air through the barn like a fan would. 
> The reason is higher energy, the effect of the efficient gain in 
> velocity, remembering that momentum is ½mv^2 ).
>
> The blurted hole is far better at getting air into a combustion 
> chamber with velocity and direction. The tools are home made. You may 
> have to reduce the hole size, it is so much more effective than a 
> drilled, sharp-edged hole. You can 'point' the hole by tilting the rod 
> as it is hammered through.
>
> For those with a technical bent, look up the 'elongation' rating for 
> the material you are using -- say, 0.20. Calculate the final diameter 
> of the outlet. Multiply the diameter by (1-elongation) i.e. Ø*0.8. 
>  That is the smallest hole you can start with to avoid splitting the 
> end of the material.
>
> For artisanal stoves, all secondary air entry holes should be shaped 
> in this manner. Full stop.
>
> Regards
>
> Crispin roaming
>
> The other night I started the charcoal for our grill badly and we 
> ended up with a feeble charcoal fire for grilling chicken.  I have a 
> BioLite campstove and the portable grill for it, so I started the 
> BioLite and transferred our chicken to it.  We cooked the rest of our 
> dinner on American Sycamore twigs.
>
> The BioLite campstove, as you know, forces air through the combustion 
> chamber using an electric blower.  If you feed the fuel carefully, 
> combustion occurs in a spiral of red-orange flame, and scarcely any 
> smoke is produced.
>
> I'm not entirely sure how the spiral is produced, but it looks to me 
> like the forced air enters the stainless "jacket" around the 
> combustion chamber circumferentially, circles the chamber, and enters 
> the chamber through the air ports with some momentum parallel to the 
> chamber sides.
>
> So it is the momentum of the air, not the shape or arrangement of air 
> ports, that produces the spiral.
>
> I was inspired by the spiral of flame in the BioLite campstove to try 
> to create a spiral of flame in my natural-draft TLUD that will help to 
> complete the fuel/air mixing and clean up the combustion.  To induce 
> the spiral, I cut some sheet-metal strakes from 26-gauge mild steel 
> and installed them between the inner & outer cans of my can stove.
>
> I have attached some photos of my stove and strakes under construction.
>
> My first experiment with the straked stove, using wood pellets as the 
> fuel, produced a central column of flame, blue at the bottom, yellow 
> at the top, that left soot on the stainless bowl of water that I 
> topped the chimney with.  I noticed a few qualitative differences from 
> prior tests.
>
> The diameter of the flame was greater than usual.  The stove seemed to 
> bring the water to a rolling boil much faster than usual.  The stove 
> also made a hissing sound, presumably because of increased turbulence.
>
> I don't remember hearing that sound from this stove before.
>
> I ran a couple of experiments with natural fuel (broken-up twigs) and 
> one with less pellets than the first.  Each of the tests produced less 
> flame than the first, and I had to restart each of the natural-fuel 
> burns at least once.  I think that I used too much wax paper to start 
> these tests, and the layer of char left by the paper blocked the draft.
>
> There wasn't any hiss in any of these experiments.
>
> I ran another experiment, tonight, using the same amount of fuel as 
> the other night (101 grams wood pellets, the top layer of which 
> consisted of pellets soaked in 91% isopropyl alcohol to aid starting), 
> but with a couple of changes to the stove.  I removed the steel wire 
> loop from the chimney.  I also removed the fan-shaped insert (shown in 
> an attached
>
> photo) from the bottom of the fuel chamber.  Conditions were also
>
> different: windy, with a rainstorm starting during my test.  This was 
> a less vigorous burn than the first one.  I could not detect any hiss.
>
> If the strakes induce any swirl, it is very subtle.  Perhaps more 
> strakes, or strakes at a different angle will produce a more powerful 
> effect.  (My strakes rise 5.8 cm in 9.8 cm, measured around the outer 
> can, diameter = 10.7 cm.)  The dramatic burn in the first experiment 
> may have been due to windless conditions.
>
> Dave
>
> --
>
> David Young
>
> dyoung at pobox.com <mailto:dyoung at pobox.com> Urbana, IL    (217) 721-9981
>
>
>
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